The present invention relates generally to vehicle steering systems and, more particularly, to a method and system for anti-static steering for vehicle steering systems.
In vehicular applications, a typical four-wheel steering system steers both the front and the rear wheels of a vehicle. More specifically, a rear wheel steering portion of the four-wheel system may produce a desired rear wheel steering angle in order to improve directional stability at high speeds and maneuverability at low speeds. A high-speed steering method implemented with this system may reduce vehicle yaw by steering the front and rear wheels in the same direction, or in phase, and thereby improve vehicle safety. In contrast, a low speed steering method implemented with four-wheel steering may achieve tight turning radii at low speeds by steering the front and rear wheels in different directions, or out of phase, to thereby reduce the effective turning radius of the vehicle.
In the rear wheel steering portion of a four-wheel steering system, an electric motor is typically employed to steer the rear wheels. Attempting to turn the rear wheels when the vehicle is substantially stopped (or when the steered wheels are otherwise static and not rolling) places high torque loads on the electric motor. This is referred to as “static steering.” Accommodating such high torque loads mandates the utilization of larger motors, requiring larger size and weight, and may result in less precise control.
On the one hand, certain steering systems may in fact provide sufficient torque so as to allow the rear wheels to be turned while the vehicle is not moving. Delphi's production QUADRASTEER™ system is one example of such a steering system. Because systems of this type are powerful enough to enable static steering, they generally do not incorporate measures to avoid or prevent static steering.
However, other newer active rear steering systems have been designed to work on passenger vehicles and, as a result, have been designed with only enough power to steer the rear wheels during dynamic maneuvers (i.e., maneuvers that take place at speeds significantly above static steering speeds). For systems of this type, there is therefore a need for properly controlling the rear wheels to avoid attempting static steering and prevent high current in the low-power motor. In this manner, any overheating, demagnetization, and/or other damage that might cause a permanent system failure may be avoided.